Torque sensor

ABSTRACT

A torque sensor which can detect a torque with high accuracy is provided. First structure and second structure are connected by a plurality of third structures. First and second strain sensors are connected between the first structure and the second structure. Each of the first and second strain sensors includes a strain body connected between the first structure and the second structure and a plurality of sensor elements provided on the strain body. The sensor elements are disposed in a region of one side of each of the first structure and the second structure with respect to a longitudinal central portion of the strain body, and the region on the one side is a region where there is only a little difference in strain between along a torque direction of the strain body and a torque-excepted direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/JP2019/004752, filed on Feb. 8, 2019, which claims priority to andthe benefit of Japanese Patent Application No. 2018-063828, filed onMar. 29, 2018. The disclosures of the above applications areincorporated herein by reference.

FIELD

The present disclosure relates to a torque sensor to be provided on, forexample, an articulation of a robot arm.

BACKGROUND

A torque sensor includes a first structure to which torque is applied, asecond structure from which torque is output, and a plurality of strainsections serving as beams configured to couple the first structure andthe second structure to each other, and a plurality of strain gagesserving as sensor elements are arranged on these strain sections. Abridge circuit is constituted of these strain gages (refer to, forexample, Patent Literature 1 (JP 2013-096735 A), Patent Literature 2 (JP2015-049209), and Patent Literature 3 (JP 2017-172983 A)).

SUMMARY

It is necessary for a bridge circuit of a torque sensor to be configuredto output a voltage for force in the torque direction, and not to outputa voltage for force in the torque-excepted direction. However, this isbased on the prerequisite that the structure of the torque sensor isprocessed precisely and a strain gauge is disposed precisely at apredetermined position of the structure. Thus, it depends on theaccuracy of processing the structure and the displacement accuracy ofthe strain gauge with respect to the structure.

An object of the embodiments is to provide a torque sensor which candetect a torque with high accuracy without depending on the accuracy ofprocessing the structure and the displacement accuracy of the straingauge with respect to the structure.

According to an embodiment, there is provided a torque sensor comprisinga first structure, a second structure, a plurality of third structuresconnecting the first structure and the second structure, and at leastone strain sensor connected between the first structure and the secondstructure, wherein the at least one strain sensor comprises a strainbody connected between the first structure and the second structure anda plurality of sensor elements provided on the strain body, theplurality of sensor elements are disposed in a region of one side ofeach of the first structure and the second structure with respect to alongitudinal central portion of the strain body, and the region on theone side is a region where there is only a little difference in strainbetween along a torque direction of the strain body and atorque-excepted direction.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a plan view showing a torque sensor to which each ofembodiments is applied.

FIG. 2 is a plan view showing FIG. 1 part of which is excluded.

FIG. 3 is a plan view showing the torque sensor of FIG. 2 part of whichis excluded and according to a first embodiment.

FIG. 4 is a perspective view of FIG. 3.

FIG. 5 is a plan view showing the enlarged part A indicated by a brokenline in FIG. 3.

FIG. 6A is a plan view shown for explaining an operation of a case whereforce in the torque (Mz) direction is applied to the torque sensor shownin FIG. 5.

FIG. 6B is a side view shown for explaining an operation of a case whereforce in the torque-excepted (Fz, Mx) direction is applied to the torquesensor shown in FIG. 5.

FIG. 7 is a perspective view showing the structure shown in FIG. 5.

FIG. 8A is a cross-sectional view along line VIIIA-VIIIA shown in FIG.7, and is a view for explaining second moment of area in thetorque-excepted (Fz, Mx) direction.

FIG. 8B is a cross-sectional view along line VIIIB-VIIIB shown in FIG.7, and is a view for explaining second moment of area in thetorque-excepted (Fz, Mx) direction.

FIG. 8C is a view shown for explaining second moment of area of ageneral structure.

FIG. 8D is a view shown for explaining second moment of area of astructure different from FIG. 8C.

FIG. 8E is a view for explaining second moment of area in the torque(Mz) direction of FIG. 8A.

FIG. 8F is a view shown for explaining second moment of area in thetorque (Mz) direction of FIG. 8B.

FIG. 8G is a view shown for explaining second moment of area of astructure different from FIG. 8C and FIG. 8D.

FIG. 8H is a view shown for explaining a positional relationship betweena structure and strain body.

FIG. 9 is a plan view showing a torque sensor according to a comparativeexample of the first embodiment.

FIG. 10A is a plan view shown for explaining an operation of a casewhere force in the torque (Mz) direction is applied to the torque sensorshown in FIG. 9.

FIG. 10B is a side view shown for explaining an operation of a casewhere force in the torque-excepted (Fz, Mx) direction is applied to thetorque sensor shown in FIG. 9.

FIG. 11 is a view showing the strain of a case where the same force isapplied in each of the axial directions of each of the torque sensor ofthe first embodiment and torque sensor of the comparative example.

FIG. 12 is a plan view showing a second embodiment and showing a firststrain sensor and second strain sensor.

FIG. 13 is a circuit diagram showing an example of a bridge circuit ofthe first strain sensor.

FIG. 14 is a view shown for explaining the states of a strain body inthe case where force in the torque direction is applied to a torquesensor of the second embodiment and in the case where force in thetorque-excepted direction is applied thereto.

FIG. 15 is a view schematically showing a torque sensor according to acomparative example of the second embodiment.

FIG. 16 is a plan view showing a third embodiment and showing the partindicated by B of FIG. 1 in an enlarging manner.

FIG. 17A is a view showing the operation of a stopper and schematicallyshowing part of FIG. 16.

FIG. 17B is a view showing the operation of a stopper different fromFIG. 17A and schematically showing part of FIG. 16.

FIG. 18 is a view shown for explaining a relationship between the torqueapplied to the torque sensor and operation of the stopper.

FIG. 19 is a view showing a relationship between the strain of thestrain gage and stress.

FIG. 20 is a plan view showing a first modification example of the thirdembodiment and showing part of the first modification example in anenlarging manner.

FIG. 21 is a plan view showing a second modification example of thethird embodiment.

DETAILED DESCRIPTION

The Hereinafter embodiments of the present invention will be describedbelow with reference to the accompanying drawings. In the drawings, thesame parts are denoted by the same reference symbols.

FIG. 1 shows an example of a torque sensor 10 to which this embodimentis applied.

In FIG. 1, the torque sensor 10 comprises a first structure 11, secondstructure 12, plurality of third structures 13, fourth structure 14,fifth structure 15, stoppers 16 and 17, and cover 18.

Each of the first structure 11 and second structure 12 is formed into anannular shape and the diameter of the second structure 12 is less thanthe diameter of the first structure 11. The second structure 12 isarranged concentric with the first structure 11, and first structure 11and second structure 12 are coupled to each other by the thirdstructures 13 serving as a plurality of radially arranged beam sections.The second structure includes a hollow section 12 a, and, for example,wiring not shown is passed through the hollow section 12 a.

The first structure 11 is coupled to, for example, an object to bemeasured, and the plurality of third structures 13 transmit torque fromthe first structure 11 to the second structure 12. Conversely, thesecond structure 12 may be coupled to the object to be measured, andtorque may be transmitted from the second structure 12 to the firststructure 11 through the plurality of third structures 13.

Although the first structure 11, second structure 12, and plurality ofthird structures 13 are constituted of metal, for example, stainlesssteel, materials other than metal can also be used if mechanicalstrength can sufficiently be obtained with respect to the appliedtorque.

FIG. 2 shows the state where the stoppers 16 and 17 of FIG. 1 areremoved. Between the first structure 11 and second structure 12, a firststrain sensor 19 and second strain sensor 20 are provided. That is, aswill be described later, one end of each of the first strain sensor 19and second strain sensor 20 is joined to the first structure 11, and theother end of each of the first strain sensor 19 and second strain sensor20 is joined to the second structure 12.

Further, the first strain sensor 19 and second strain sensor 20 arearranged at positions symmetrical with respect to the center (center ofaction of torque) of each of the first structure 11 and second structure12. In other words, the first strain sensor 19 and second strain sensor20 are arranged on the diameters of the annular first structure 11 andsecond structure 12.

A thickness of each of the first strain sensor 19 and second strainsensor 20, i.e., thickness of a strain body to be described later isless than the thickness of the third structure 13. The mechanicalstrength of the torque sensor 10 is set according to the thickness andwidth of the third structure 13. The strain body is provided with aplurality of strain gages functioning as sensor elements, and a bridgecircuit is constituted of these sensor elements.

Each of the stoppers 16 and 17 has a function of protecting each of thefirst strain sensor 19 and second strain sensor 20 from mechanicaldeformation, and serving as a cover of each of the first strain sensor19 and second strain sensor 20. Details of the stoppers 16 and 17 willbe described later.

The first strain sensor 19 is connected to a flexible board 21 andsecond strain sensor 20 is connected to a flexible board 22. Theflexible boards 21 and 22 are connected to a printed board (not shown)covered with a cover 18. On the printed board, an operational amplifierand the like configured to amplify an output voltage of the bridgecircuit to be described later are arranged. The circuit configuration isnot the nature of this embodiment, and a description thereof is omitted.

First Embodiment

FIG. 3 and FIG. 4 are views showing a first embodiment, are formed byremoving the first strain sensor 19, second strain sensor 20, flexibleboards 21 and 22, cover 18 and the like from FIG. 1 and FIG. 2, and showonly the first structure 11, second structure 12, plurality of thirdstructures 13, fourth structure 14, and fifth structure 15.

The first embodiment is configured to have such a structure that, whenforce in the direction other than the torque direction Mz, particularly,in the Fz direction or Mx direction indicated by the arrow shown in FIG.3 is applied to the torque sensor 10, strain is not concentrated at theplurality of strain gages serving as sensor elements and provided on thestrain bodies of the first strain sensor 19 and second strain sensor 20.

More specifically, the fourth structure 14 and fifth structure 15 areprovided at positions symmetrical with respect to the center of each ofthe first structure 11 and second structure 12, fourth structure 14includes a concave section 14 f continuous from the first structure 11to the second structure 12, and fifth structure includes a concavesection 15 f continuous from the first structure 11 to the secondstructure 12. As will be described later, the first strain sensor 19 isarranged inside the concave section 14 f of the fourth structure 14, andsecond strain sensor 20 is arranged inside the concave section 15 f ofthe fifth structure 15.

It should be noted that although the first embodiment is shown about thecase where two strain sensors including the first strain sensor 19 andsecond strain sensor 20 are provided, the number of strain sensors maybe three or more. In this case, it is sufficient if the number ofstructures is increased according to the number of strain sensors.

The fourth structure 14 and fifth structure 15 are identical in thestructure, and hence only the fourth structure 14 will specifically bedescribed below.

As shown in FIG. 5, the fourth structure 14 includes a first connectionsection 14 a and second connection section 14 b serving as a jointsection configured to join the first strain sensor 19, third connectionsection 14 c and fourth connection section 14 d serving as a beam, andopening 14 e surrounded by the first connection section 14 a, secondconnection section 14 b, third connection section 14 c, and fourthconnection section 14 d.

In other words, the fourth structure 14 is a beam including the opening14 e provided between the first structure 11 and second structure 12.

The first connection section 14 a extends from the first structure 11 tothe second structure 12 side. The second connection section 14 b extendsfrom the second structure 12 to the first structure 11 side.

The third connection section 14 c and fourth connection section 14 dserving as the beam are provided between the first connection section 14a and second connection section 14 b.

The length L1 of each of the third connection section 14 c and fourthconnection section 14 d is shorter than the length L2 (shown also inFIG. 1) of the third structure 13 serving as the beam. The width W1 ofeach of the third connection section 14 c and fourth connection 14 d inthe torque (Mz) direction is narrower than the width W2 of each of thefirst connection section 14 a and second connection section 14 b in thetorque direction, the total of the widths W1 of the third connectionsection 14 c and fourth connection section 14 d is narrower than thewidth W3 (shown in FIG. 1) of the third structure 13 in the torque (Mz)direction. For this reason, the stiffness of the third connectionsection 14 c and fourth connection section 14 d in the torque directionis lower than the stiffness of each of the first connection section 14a, second connection section 14 b, and third structure 13 in the torquedirection.

Further, the thickness of each of the third connection section 14 c andfourth connection section 14 d in the Fz direction is equal to thethickness of each of the first structure, second structure, and thirdstructure in the Fz direction. Furthermore, the total of the length L11of the first connection section 14 a, length L12 of the secondconnection section 14 b, and length L1 of the third connection section14 c or fourth connection section 14 d is equal to the length of thethird structure 13. Accordingly, the stiffness of the third connectionsection 14 c and fourth connection section 14 d in the Fz directionbecomes somewhat less than the stiffness of the third structure 13 inthe Fz direction.

That is, as shown in FIG. 6A to be described later, in the torque (Mz)direction, the first connection section 14 a and first structure 11constitute a high-stiffness section HS1, and second connection section14 b and second structure 12 constitute a high-stiffness section HS2.Furthermore, in the torque (Mz) direction, the third connection section14 c constitutes a low-stiffness section LS1, and fourth connectionsection 14 d constitutes a low-stiffness section LS2.

It should be noted that the total of the length L11 of the firstconnection section 14 a, length L12 of the second connection 14 b, andlength L1 of the third connection section 14 c or fourth connectionsection 14 d is not limited to the case where the total is equal to thelength of the third structure 13, and may not be equal to the length ofthe third structure 13.

The first connection section 14 a includes the aforementioned concavesection 14 f. The thickness of the whole part of the concave section 14f is less than the thickness of each of the first to third structures11, 12, and 13.

One end of the first strain sensor 19 is connected to the concavesection 14 f of the first connection section 14 a, and the other endthereof is connected to the concave section 14 f of the secondconnection section 14 b. Accordingly, the first strain sensor 19 liesastride the opening 14 e. The bottom of the concave section 14 f ispositioned lower than the center of the thickness of the fourthstructure 14, and the upper surface of the strain body constituting thefirst strain sensor 19 is made flush with a plane including the centerof gravity of a structure constituted of the first structure 11, secondstructure 12, plurality of third structures 13, fourth structure 14, andfifth structure 15.

FIG. 6A and FIG. 6B are views schematically showing FIG. 5, FIG. 6Ashows a case where force in the torque (Mz) direction is applied to thetorque sensor 10, and FIG. 6B shows a case where force in thetorque-excepted (Fz, Mx) direction is applied to the torque sensor 10.

As shown in FIG. 6A, when force in the torque (Mz) direction is appliedto the torque sensor 10, the third connection section 14 c and fourthconnection section 14 d functioning as the low-stiffness sections LS1and LS2 become deformed, whereby the first strain sensor 19 (secondstrain sensor 20) becomes deformed, and the torque can be detected.

On the other hand, as shown in FIG. 6B, when force in thetorque-excepted (Fz, Mx) direction is applied to the torque sensor 10,i.e., when the first structure 11 is displaced in the directionindicated by the arrow in FIG. 6B with respect to the second structure12, the stiffness of the first connection section 14 a and secondconnection section 14 b, and stiffness of the third connection section14 c and fourth connection section 14 d are approximately equal to eachother. Accordingly, the total length L2 of the length L11 of the firstconnection section 14 a, length L12 of the second connection section 14b, and length L1 of the third connection section 14 c or fourthconnection section 14 d acts as the effective length. The length L2 islonger than the length L1 of the third connection section 14 c or fourthconnection section 14 d, and hence when force in the torque-excepted(Fz, Mx) direction is applied, deformation of the first strain sensor 19(second strain sensor 20) takes place within the range of the length L2,it is possible to prevent the strain from being concentrated at theplurality of strain gages serving as the sensor elements provided on thestrain body of the first strain sensor 19, and it is possible to preventthe detection accuracy of the first strain sensor 19 (second strainsensor 20) from being deteriorated.

FIG. 7 is a view schematically showing the fourth structure 14. Thesecond moment of area (deformability) of the fourth structure 14 andrequirements for the fourth structure 14 (fifth structure 15) will bedescribed below with reference to FIG. 7.

It is assumed here that second moment of area of a case where thehigh-stiffness section HS2 of the fourth structure 14 is fixed and forcein the torque (Mz) direction is applied to the high-stiffness sectionHS1 is expressed by Js, second moment of area of a case where force inthe torque (Mz) direction is applied to the low-stiffness sections LS1and LS2 is expressed by Jw, second moment of area of a case where forcein the torque-excepted (Fz) direction is applied to the high-stiffnesssection HS1 is expressed by Is, and second moment of area of a casewhere force in the torque-excepted (Fz) direction is applied to thelow-stiffness sections LS1 and LS2 is expressed by Iw.

The ratio between the second moment of area of the high-stiffnesssection HS1 in the torque (Mz) direction and second moment of area ofthe low-stiffness sections LS1 and LS2 is expressed by the followingformula (1).Js/Jw  (1)

The ratio between the second moment of area of the high-stiffnesssection HS1 in the torque-excepted (Fz) direction and second moment ofarea of the low-stiffness sections LS1 and LS2 is expressed by thefollowing formula (2).Is/Iw  (2)

When each of the values of the formulas (1) and (2) is “1,” the secondmoment of area of the high-stiffness section HS1 and second moment ofarea of the low-stiffness sections LS1 and LS2 are equal to each other,and deformation is not concentrated at the low-stiffness sections LS1and LS2. The greater each of the values of the formulas (1) and (2) than“1,” the higher the concentration of deformation at the low-stiffnesssections LS1 and LS2 is.

In order to concentrate the strain at the plurality of strain gagesserving as sensor elements provided on the strain body of the firststrain sensor 19 in the case where force in the torque (Mz) direction isapplied, and in order to displace the strain-concentrated position fromthe strain gages in the case where force in the torque-excepted (Fz, Mx)direction is applied, it is desirable that the degree of deformationconcentration (α) on one side be close to 1 (α→1), and degree ofdeformation concentration (β) on the other side be enormously greaterthan the degree of deformation concentration (α) (β>>α).

When the degree of deformation concentration at the low-stiffnesssections LS1 and LS2 in the case where force in the torque (Mz)direction is applied is greater than the degree of deformationconcentration at the low-stiffness sections LS1 and LS2 in the casewhere force in the torque-excepted (Fz) direction is applied, the fourthstructure 14 (fifth structure 15) is easily deformable with respect tothe force in the torque direction, and is hardly deformable in thetorque-excepted direction. That is, it is the condition required of thefourth structure 14 (fifth structure 15) that the relationship indicatedby the following formula (3) be established.Js/Jw>Is/Iw  (3)

More specifically, FIG. 8A is a cross-sectional view along lineVIIIA-VIIIA shown in FIG. 7, and shows an example of the dimensions ofthe high-stiffness section HS1. FIG. 8B is a cross-sectional view alongline VIIIB-VIIIB shown in FIG. 7, and shows an example of the dimensionsof the low-stiffness sections LS1 and LS2.

As shown in FIG. 8A, the second moment of area Is concerning the axisN1-N1 of a case where force in the torque-excepted (Fz) direction isapplied to the high-stiffness section HS1 having a U-shaped crosssection is as follows. Here, the axis N1-N1 is an axis passing throughthe center of the high-stiffness section HS1 in the thickness direction.

As shown in FIG. 8C, in general, when dimensions of both a structurehaving an L-shaped cross section and structure having a U-shaped crosssection satisfy the relationships of b=B−a, h=e₁−t, the second moment ofarea Is of the structure having the L-shaped cross section and secondmoment of area Is of the structure having the U-shaped cross section areequal to each other, and are expressed by the following formula (4).Is=(Be ₁ ³ −bh ³ +ae ₂ ³)/3  (4)Here, h=e ₁ −t,e ₁=(aH ² +bt ²)/(2(aH+bt))e ₂ =H−e ₁

Accordingly, the second moment of area Is concerning the axis N1-N1 inthe case where force in the torque-excepted (Fz) direction is applied tothe high-stiffness section HS1 shown in FIG. 8A can be obtained by theformula (4).

It should be noted that e₁ is the position of the center of gravity inthe structure functioning as an elastic body and constituted of thefirst structure 11, second structure 12, plurality of third structures13, fourth structure 14, and fifth structure 15, and is half thethickness of the structure. Accordingly, e₁ becomes approximately 6 withrespect to the thickness H (=12) (H=12, e₁≈6). Accordingly, e₂ isapproximately 6 (e₂≈6 is obtained).

When the dimensions shown in FIG. 8A are substituted into the formula(4), the following result is obtained.

Is = (Be₁³ − bh³ + ae₂³)/3   = (14 × 6³ − 8 × (6 − 5.8)³ + 6 × 6³)/3   = 1440

Further, as shown in FIG. 8B, the second moment of area Is concerningthe axis N2-N2 of a case where force in the torque-excepted (Fz)direction is applied to the low-stiffness sections LS1 and LS2 each ofwhich has a rectangular cross section is as follows. Here, the axisN2-N2 is an axis passing through the centers of the low-stiffnesssections LS1 and LS2 in the thickness direction.

As shown in FIG. 8D, in general, the second moment of area Iw′ of astructure having a rectangular cross section is expressed by thefollowing formula (5).Iw′=bh ³/12  (5)

When the dimensions shown in FIG. 8B are substituted into the formula(5), the following result is obtained.

Iw^(′) = 2 × 12³/12    = 288

The low-stiffness sections LS1 and LS2 shown in FIG. 8B have tworectangular cross sections in all, and hence the second moment of areaIw in the torque-excepted (Fz) direction concerning the axis N2-N2 isexpressed by the following formula (6).Iw=2×Iw′  (6)

Accordingly, the second moment of area in the torque-excepted (Fz)direction concerning the axis N2-N2 is as follows.Iw=576

On the other hand, as shown in FIG. 8E, when force in the torque (Mz)direction is applied to the high-stiffness section HS1 having a U-shapedcross section, the second moment of area Js concerning the axis N3-N3 isas follows. Here, the axis N3-N3 is an axis passing through the centerof the high-stiffness section HS1 in the width direction.

As shown in FIG. 8G, in general, when dimensions of a structure havingan I-shaped cross section and structure having a U-shaped cross sectionsatisfy the relationships of b=B−a, h=H−2t, the second moment of area ofthe structure having the I-shaped cross section and second moment ofarea of the structure having the U-shaped cross section are equal toeach other, and are expressed by the following formula (7).Js=(BH ³ −bh ³)/12  (7)

When the dimensions shown in FIG. 8A are substituted into the formula(7), the following result is obtained.

$\begin{matrix}{{Js} = {\left( {{12 \times 14^{3}} - {{6.2} \times 8^{3}}} \right)/12}} \\{= 2479}\end{matrix}$

Further, as shown in FIG. 8F, when force in the torque (Mz) direction isapplied to the low-stiffness sections LS1 and LS2 having rectangularcross sections, the second moment of area Jw′ concerning the axis N4-N4is expressed by the following formula (8) as previously described inconnection with FIG. 8D. Here, the axis N4-N4 is an axis passing throughthe center of the low-stiffness section LS1 in the width direction.Jw′=bh ³/12  (8)

When the dimensions shown in FIG. 8B are substituted into the formula(8), the following result is obtained.

Jw^(′) = 12 × 2³/12    = 8

The low-stiffness sections LS1 and Ls2 shown in FIG. 8F have tworectangular cross sections in all, and hence the second moment of areaJw in the torque (Mz) direction concerning the axis N4-N4 is expressedby the following formula (9).Jw=2×Jw′  (9)

Accordingly, the second moment of area Iw in the torque-excepted (Fz)direction concerning the axis N2-N2 is as follows.Jw=16

When the second moment of area Is (=1440) in the torque-excepted (Fz)direction, Iw (=576), second moment of area Js (=2479) in the torque(Mz) direction, and Jw (=16) which are obtained in the aforementionedmanner are substituted into the above formula (3), the following resultis obtained and it can be seen that the condition of the formula (3) issatisfied.Js/Jw>Is/Iw2479/16>1440/576155>2.5

Accordingly, it can be seen that the fourth structure 14 and fifthstructure 15 are easily deformable with respect to the force in thetorque (Mz) direction, and are hardly deformable with respect to theforce in the torque-excepted (Fz) direction.

FIG. 8H shows a positional relationship between the concave section 14 fand first strain sensor 19 (strain body). As described previously, thebottom of the concave section 14 f is positioned at the center H/2 ofthe thickness of the fourth structure or lower. More specifically, inorder to make the upper surface of the strain body constituting thefirst strain sensor 19 flush with the plane CG including the center ofgravity of the structure constituted of the first structure 11, secondstructure 12, plurality of third structures 13, fourth structure 14, andfifth structure 15, the bottom of the concave section 14 f is set atposition lower than the plane CG including the center of gravity of thefourth structure by the thickness of the strain body. This position isin the neutral plane, and neither compressive force nor tensile force isexerted on the strain body at this position. Accordingly, it is possibleto reduce the strain in the bend direction of the strain body, i.e.,strain in the torque-excepted (Fz) direction.

Advantage of First Embodiment

According to the first embodiment, each of the fourth structure 14provided with the first strain sensor 19 and fifth structure 15 providedwith the second strain sensor 20 is provided with the first connectionsection 14 a and second connection section 14 b functioning as thehigh-stiffness sections with respect to the force in torque (Mz)direction and torque-excepted (Fz, Mx) direction, and third connectionsection 14 c and fourth connection section 14 d functioning as thelow-stiffness sections with respect to the force in the torque (Mz)direction and functioning as the high-stiffness sections with respect tothe force in the torque-expected (Fz, Mx) direction. Accordingly, it ispossible to prevent the strain caused by the force in thetorque-excepted direction from being concentrated at the strain gages51, 52, 53, and 54 of each of the first strain sensor 19 and secondstrain sensor 20. Accordingly, it is possible to reduce the absoluteamount of the strain to be applied to the strain gages 51, 52, 53, and54, and greatly reduce the detection voltage of each of the first strainsensor 19 and second strain sensor 20 corresponding to the force in thetorque-excepted direction. Accordingly, it is possible to provide ahigh-accuracy torque sensor capable of preventing torque axisinterference and torque-excepted axis interference from occurring andpreventing the size thereof from being increased.

Hereinafter, the advantage of the first embodiment will be describedwith reference to a comparative example.

FIG. 9 shows a comparative example of the torque sensor 10. A torquesensor 30 shown in FIG. 9 differs from the torque sensor 10 shown in thefirst embodiment in the configuration of each connection section of thefirst strain sensor 19 and second strain sensor 20, and the otherconfigurations are identical to the first embodiment.

In the torque sensor 30, one end of each of the first strain sensor 19and second strain sensor 20 is connected to a projection 11-1 providedon the first structure 11, and the other end of each of the strainsensors 19 and 20 is connected to a projection 12-1 provided on thesecond structure 12. Each of the projections 11-1 and 12-1 has athickness equal to, for example, the first structure 11 and secondstructure 12. The gap between the projection 11-1 and projection 12-1 isequal to the length L1 of each of the third connection section 14 c andfourth connection section 14 d shown in FIG. 5.

In the torque sensor 30 as the comparative example, only the thirdstructure 13 functions as a high-stiffness section with respect to theforce in the torque direction and torque-excepted direction, and each ofthe first strain sensor 19 and second strain sensor 20 is only providedwith a strain body between the first structure 11 and second structure12. Accordingly, in each of the case where force in the torque (Mz)direction is applied to the torque sensor 30, and case where force inthe torque-excepted (Fz, Mx) direction is applied to the torque sensor30, strain is concentrated at the strain gages provided on the strainbody of each of the first strain sensor 19 and second strain sensor 20.

FIG. 10A and FIG. 10B are views each schematically showing FIG. 9, FIG.10A shows a case where force in the torque (Mz) direction is applied tothe torque sensor 30, and FIG. 10B shows a case where force in thetorque-excepted (Fz, Mx) direction is applied to the torque sensor 30.

FIG. 11 shows the strain of a case where the same force is applied toeach of the torque sensor 10 according to the first embodiment andtorque sensor 30 according to the comparative example in each of theaxial directions.

As is evident from FIG. 11, in the case of the torque sensor 10according to the first embodiment, the strain corresponding to the forcein the torque (Mz) direction is greater than the comparative example,and strain corresponding to the force in the torque-excepted (Fx, Fy,Fz, Mx, My) direction is less than the comparative example.Particularly, it can be seen that it is possible to make the straincorresponding to the force in each of the Fz and Mx directionsremarkably smaller than the comparative example. Therefore, according tothe first embodiment, it is possible to reduce the strain caused by theforce in the torque-excepted direction in the first strain sensor 19 andsecond strain sensor 20, and prevent the detection accuracy of the firststrain sensor 19 and second strain sensor 20 from being deteriorated.

Further, the upper surface of the strain body constituting the firststrain sensor 19 is positioned flush with the plane CG including thecenter of gravity of the structure constituted of the first structure11, second structure 12, plurality of third structures 13, fourthstructure 14, and fifth structure 15. Accordingly, it is possible toreduce the strain in the bend direction of the strain body, i.e., strainin the torque-excepted (Fz) direction.

Second Embodiment

FIG. 12 shows a second embodiment.

As described previously, a first strain sensor 19 is provided on afourth structure 14, and second strain sensor 20 is provided on a fifthstructure 15. Configurations of the first strain sensor 19 and secondstrain sensor 20 are identical to each other, and hence only theconfiguration of the first strain sensor 19 will be described below.

The first strain sensor 19 is provided with a strain body 41, andplurality of strain gages 51, 52, 53, and 54 serving as sensor elementsarranged on the surface of the strain body 41.

The strain body 41 is constituted of a rectangular metallic plate, forexample, stainless steel (SUS). The thickness of the strain body 41 isless than the thickness of the third structure 13.

Each of the strain gages 51, 52, 53, and 54 is constituted of, forexample, a Cr—N thin-film resistive element provided on the strain body41. The material for the thin-film resistive element is not limited toCr—N.

The strain body 41 is connected to a first connection section 14 a atone end thereof, and is connected to a second connection section 14 b atthe other end thereof. As the method of connecting the strain body 41 tothe first connection section 14 a and second connection section 14 b,for example, a connection method using welding, screwing or an adhesivecan be employed.

In the strain body 41, for example, a part thereof between a position atwhich the strain body 41 is welded to the first connection section 14 aand position at which the strain body 41 is welded to the secondconnection section 14 b functions as the substantial strain body.Accordingly, the effective length of the strain body 41 corresponds tothe length from the position at which the strain body 41 is connected tothe first connection section 14 a to the position at which the strainbody 41 is connected to the second connection section 14 b.

The plurality of strain gages 51, 52, 53, and 54 are arranged on thestrain body 41 in the area AR1 on the second structure 12 side of thecentral part CT of the effective length of the strain body 41. This areaAR1 is within the range of an opening 14 e, and is an area in whichgreat strain is caused to the strain body 41. As will be describedlater, this area AR1 is an area in which the sensitivity of the firststrain sensor 19 to the force in the torque-excepted direction, forexample, Fx or My direction and sensitivity of the first strain sensor19 to the force in the torque (Mz) direction become identical to eachother.

The strain gages 51, 52, 53, and 54 are arranged in the area AR1 in sucha manner that the longitudinal direction of each of the strain gages 51,52, 53, and 54 is made along two diagonal lines DG1 and DG2 of thestrain body 41. That is, the strain gages 51 and 52 are arranged in sucha manner that the longitudinal direction of each of the strain gages 51and 52 is set along one diagonal line DG1 indicated by a broken line,and strain gages 53 and 54 are arranged in such a manner that thelongitudinal direction of each of the strain gages 53 and 54 is setalong the other diagonal line DG2 indicated by a broken line. Thediagonal lines DG1 and DG2 each correspond to a rectangular areapositioned within the opening 14 e of the strain body 41.

The strain gages 51, 52, 53, and 54 of the first strain sensor 19constitute one bridge circuit, and strain gages 51, 52, 53, and 54 ofthe second strain sensor 20 also constitute one bridge circuit.Accordingly, the torque sensor 10 comprises two bridge circuits.

FIG. 13 shows an example of a bridge circuit 50 of the first strainsensor 19. The second strain sensor 20 also comprises a bridge circuitof a configuration identical to the bridge circuit 50. Each of an outputvoltage of the bridge circuit 50 of the first strain sensor 19 andoutput voltage of the bridge circuit 50 of the second strain sensor 19is compensated for an offset, temperature or the like thereof by using,for example, software not shown. Thereafter, the output voltage of thebridge circuit 50 of the first strain sensor 19 and output voltage ofthe bridge circuit 50 of the second strain sensor 19 are unified intoone result and the result is output as a detection voltage of the torquesensor 10. Compensation for an offset, temperature or the like is notlimited to software, and is enabled by hardware.

In the bridge circuit 50, a series circuit of the strain gage 52 andstrain gage 53 and series circuit of the strain gage 54 and strain gage51 are each arranged between the power source Vo and ground GND. Anoutput voltage Vout+ is output from a connection node of the strain gage52 and strain gage 53, and output voltage Vout− is output from aconnection node of the strain gage 54 and strain gage 51. The outputvoltage Vout+ and output voltage Vout− are supplied to an operationalamplifier OP, and an output voltage Vout is output from an output end ofthe operational amplifier OP.

When force in the torque (Mz) direction is applied to the torque sensor10, the output voltage Vout of the torque sensor 10 shown by a formula(10) is obtained from the output voltage Vout+ of the one connectionnode of the bridge circuit 50 and output voltage Vout− of the otherconnection node.

$\begin{matrix}\begin{matrix}{{Vout} = \left( {{Vout} + {- {Vout}} -} \right)} \\{= {\left( {{R{3/\left( {{R2} + {R3}} \right)}} - {R{1/\left( {{R1} + {R4}} \right)}}} \right) \cdot {Vo}}}\end{matrix} & (10)\end{matrix}$

Here, R1 is a resistance value of the strain gage 51, R2 is resistancevalue of the strain gage 52, R3 is resistance value of the strain gage53, and R4 is resistance value of the strain gage 54.

In a state where no torque is applied to the torque sensor 10, ideally,R1, R2, R3, and R4 are all equal to each other and are equal to R(R1=R2=R3=R4=R). However, actually, there are variations in theresistance values and, in the state where no torque is applied to thetorque sensor 10, a voltage incidental to the variation in theresistance values is output. This voltage is made zero by offsetadjustment.

On the other hand, when force in the torque-excepted direction, e.g.,force in the Fx or My direction is applied to the torque sensor 10, theresistance values R1 to R4 are changed, whereby the output voltage Voutis output from the bridge circuit 50. However, as the output voltage ofthe bridge circuit 50 of the second strain sensor 20, a voltage having apositive/negative sign opposite to the output voltage of the bridgecircuit 50 of the first strain sensor 19 is output. Accordingly, theoutput voltages in those bridge circuits 50 are identical to each otherin absolute value and different from each other in positive/negativesign, and hence cancel each other out, and the detection voltage becomes0 volt.

It is desirable that the strain gages 51, 52, 53, and 54 functioning asthe sensor elements should output the same voltage when the amount ofdisplacement is the same in both the torque (Mz) direction andtorque-excepted (Fx, My) direction. Accordingly, it is desirable thatthe strain gages 51, 52, 53, and 54 be arranged in such an area that thestrain of the strain body 41 is the same (such an area that has the samemeasurement sensitivity) in both the torque (Mz) direction andtorque-excepted (Fx, My) direction.

FIG. 14 schematically shows the states of the strain body 41 in the casewhere force in the torque (Mz) direction is applied to the torque sensor10 and in the case where force in the torque-excepted (Fx, My) directionis applied thereto.

When the behavior of the strain body 41 provided between the firststructure 11 and second structure 12 is macroscopically observed, itseems that the strain body 41 is made to change in the shearingdirection in both the case where force in the torque (Mz) direction isapplied to the torque sensor 10 and case where force in thetorque-excepted (Fx, My) direction is applied thereto.

However, when the behavior of the strain body 41 provided between thefirst structure 11 and second structure 12 is microscopically observed,in the case where the force in the torque (Mz) direction is applied tothe torque sensor 10, turning force (torque) is exerted on the strainbody 41. On the other hand, in the case where the force in thetorque-excepted (Fx, My) direction is applied to the torque sensor 10,translational force is exerted on the strain body 41. Accordingly,deformation of the strain body 41 differs between the case where theforce in the torque (Mz) direction is applied and case where the forcein the torque-excepted (Fx, My) direction is applied.

That is, a difference is caused between the deformation of the strainbody 41 in the area AR1 on the second structure 12 side and deformationof the strain body 41 in the area AR2 on the first structure 11 side.More specifically, a difference between the strain of the strain body 41in the case where force in the torque (Mz) direction is applied in thearea AR1 of the strain body 41 and strain of the strain body 41 in thecase where force in the torque-excepted (Fx, My) direction is appliedtherein is less than a difference between the strain of the strain body41 in the case where force in the torque (Mz) direction is applied inthe area AR2 of the strain body 41 and strain of the strain body 41 inthe case where force in the torque-excepted (Fx, My) direction isapplied.

That is, in the area AR1 on the second structure 12 side, a differencebetween the strain of the strain body 41 in the case where force in thetorque (Mz) direction is applied and strain of the strain body 41 in thecase where force in the torque-excepted (Fx, My) direction is applied issmall.

Accordingly, when the plurality of strain gages 51, 52, 53, and 54 arearranged in the area AR1, a difference between the torque (Mz) detectionsensitivity and torque-excepted (fx, My) detection sensitivity is assmall as less than 1%. Conversely, when the plurality of strain gages51, 52, 53, and 54 are arranged in the area AR2, a difference betweenthe torque detection sensitivity and torque-excepted detectionsensitivity is several percent. Therefore, it is desirable that theplurality of strain gages 51, 52, 53, and 54 be arranged in the area AR1on the second structure 12 side.

Advantage of Second Embodiment

According to the second embodiment described above, each of the firststrain sensor 19 and second strain sensor 20 comprises the strain body41 connected between the first structure 11 and second structure 12, andplurality of strain gages 51, 52, 53, and 54 serving as the sensorelements provided on the strain body 41, and the plurality of straingages 51, 52, 53, and 54 are arranged in the area AR1 on the secondstructure 12 side of the central part CT of the strain body 41 in thelongitudinal direction thereof. The area AR1 of the strain body 41 is anarea in which a difference between the strain (sensitivity) (a1, a2) ofthe case where force in the torque direction is applied to each of thefirst strain sensor 19 and second strain sensor 20 and strain(sensitivity) (b1, b2) of the case where force in the torque-excepteddirection is applied thereto is small (a1≈b1, a2≈b2, a1≠a2).Accordingly, by adjusting each of the first strain sensor 19 and secondstrain sensor 20 in torque sensitivity, it is possible to prevent thetorque detection sensitivity from being deteriorated independently ofthe machining accuracy of the first structure 11, second structure 12,and third structures 13, and further independently of the arrangementaccuracy of the first strain sensor 19 and second strain sensor 20relative to the first structure 11 and second structure 12.

Moreover, in the bridge circuit 50 arranged in the area AR1 of thestrain body 41, a difference between the detection sensitivity to forcein the torque direction and detection sensitivity to force in thetorque-excepted direction is small, and hence an error in the outputvoltage of each of the first strain sensor 19 and second strain sensor20 is also small. Therefore, at the time of calibration of the voltageoutput from each of the two bridge circuits 50, it is possible to alsocalibrate the detection error in regard to torque-excepted items by onlycalibrating the detection error in regard to torque. Accordingly, thereis no need to provide a separate strain sensor in order to detect theforce in the torque-excepted (Fx, My) direction, and hence it ispossible to shorten the calibration time, and realize a high-speedresponse.

Hereinafter, the advantage of the second embodiment will specifically bedescribed.

FIG. 15 schematically shows a torque sensor 60 according to acomparative example. This torque sensor 60 comprises a first strainsensor 61 and second strain sensor 62 between a first structure 11 andsecond structure 12. Each of the first strain sensor 61 and secondstrain sensor 62 includes a strain body 63, and a plurality of straingages 51, 52, 53, and 54 constituting a bridge circuit shown in FIG. 13are arranged on each of the strain bodies 63. FIG. 15 is a schematicview, and hence third structures 13 are omitted.

In the comparative example, arrangement of the strain gages 51, 52, 53,and 54 is different from the second embodiment. That is, the straingages 52 and 53 are arranged in the area of the strain body 63 on thefirst structure 11 side, and strain gages 51 and 54 are arranged in thearea of the strain body 63 on the second structure 12 side.

In the case of the configuration shown in FIG. 15, regarding the straingages 52 and 53 arranged in the area on the first structure 11 side, thestrain of the strain body 63 differs between the torque (Mz) directionand torque-excepted (Fx, My) direction. Accordingly, the differencebetween the sensitivity of the first strain sensor 61 or sensitivity ofthe second strain sensor 62 of the case where force in the torque (Mz)direction is applied and sensitivity of the first strain sensor 61 orsensitivity of the second strain sensor 62 of the case where force inthe torque-excepted (Fx, My) direction is applied is great.

More specifically, when force in the torque-excepted (Fx, My) directionis applied to the torque sensor 60, the sensitivity in thetorque-excepted (Fx, My) direction differs from the sensitivity in thetorque (Mz) direction, and hence the value (positive value) of theoutput voltage of the first strain sensor 61 and value (negative value)of the output voltage of the second strain sensor 62 differ from eachother. Accordingly, the torque sensor 60 outputs an error constituted ofan average value of the first strain sensor 61 and second strain sensor62.

On the other hand, in the case of the torque sensor 10 of the secondembodiment, when force in the torque-excepted (Fx, My) direction isapplied to the torque sensor 10, the sensitivity in the torque-excepted(Fx, My) direction is coincident with the sensitivity in the torque (Mz)direction. Accordingly, the value (positive value) (Vout1) of the outputvoltage of the first strain sensor 19 and value (negative value)(−Vout2) of the output voltage of the second strain sensor 20 becomeapproximately equal to each other (|Vout1|≈|−Vout2|). Accordingly, theoutput voltages of the first strain sensor 61 and second strain sensor62 cancel each other out, and thus the output of the torque sensor 10becomes approximately zero. Therefore, in the case of the secondembodiment, it is possible to reduce the detection error in regard tothe force in the torque-excepted (Fx, My) direction.

In the case of the torque sensor 60 according to the comparativeexample, the error in the output voltage of each of the first strainsensor 61 and second strain sensor 62 is great (|Vout1|≠|−Vout2|)between the torque (Mz) direction and torque-excepted (Fx, My)direction. Accordingly, in order to calibrate these errors, there is aneed to carry out both of calibration for correction of the detectionerror in the torque direction and calibration for correction of thedetection error in the torque-excepted direction. Therefore, the torquesensor 60 according to the comparative example needs to be separatelyprovided with a bridge circuit including strain gages configured todetect force in the torque-excepted direction. Therefore, regarding thetorque sensor 60 according to the comparative example, the circuit boardis upsized, time of arithmetic processing to be carried out by softwareis increased, adjustment work is complicated as compared with the secondembodiment, and responsiveness is deteriorated.

On the other hand, in the case of the second embodiment, there is hardlyany error in the output voltage of each of the first strain sensor 19and second strain sensor 20 between the torque (Mz) direction andtorque-excepted (Fx, My) direction. Accordingly, it is sufficient ifonly the detection error in the torque direction is corrected.Therefore, it is possible to shorten the calibration time and improvethe responsiveness of the torque sensor.

Further, the second embodiment is not limited to the configuration ofthe torque sensor 10, and it is sufficient if the strain gages 51, 52,53, and 54 are arranged in the area AR1. Therefore, even when thearrangement according to the second embodiment is applied to, forexample, the torque sensor 30 having the configuration shown in FIG. 9,an advantage identical to the second embodiment can be obtained.

Third Embodiment

FIG. 16 is a view showing a third embodiment, and shows the partindicated by B in FIG. 1 in an enlarging manner.

As described previously with reference to FIG. 2, the first strainsensor 19 is covered with the stopper 16 and second strain sensor 20 iscovered with the stopper 17. The stopper 16 and stopper 17 are formedof, for example, stainless steel or ferrous alloy. The stopper 16 andstopper 17 prevent mechanical deformation of the first strain sensor 19and second strain sensor 20 from occurring, and protect the strain gages51, 52, 53, and 54. Furthermore, the stopper 16 and stopper 17 eachdouble as waterproof covers of the first strain sensor 19 and secondstrain sensor 20. A description of the specific waterproof structure isomitted.

The stopper 16 and stopper 17 are identical in configuration, and henceonly the stopper 16 will be described below.

As shown in FIG. 16, the stopper 16 includes a one end 16 a and theother end 16 b, and a width of the other end 16 b is made narrower thanthe width of the one end 16 a. The one end 16 a of the stopper 16 is,for example, press-fitted into the concave section 14 f serving as anengaging section formed on the second structure 12 side of the fourthstructure 14, and is fixed therein. The other end 16 b of the stopper 16is arranged inside the concave section 14 f formed on the firststructure 11 side of the fourth structure 14. The width of the other end16 b of the stopper 16 is narrower than the width of the concave section14 f provided on the first structure 11 side, and between the both sidesof the other end 16 b of the stopper 16 and side faces of the concavesection 14 f, gaps GP are provided.

The gap GP is determined according to the stiffness of the thirdstructure 13 and rated torque.

More specifically, when torque of, for example, 1000 N·m is applied tothe torque sensor 10, if the first structure 11 is deformed by, forexample, 10 μm relatively to the second structure 12, the gap GP is setto, for example, 10 μm.

FIG. 17A, and FIG. 17B are views showing the operation of the stopper,and schematically shows part of FIG. 16.

As shown in FIG. 17A, when no torque is applied to the torque sensor 10,between each of the both sides of the other end 16 b of the stopper 16and concave section 14 f, a predetermined gap GP is provided. In thisstate, when torque equal to or less than the rated torque is applied tothe torque sensor 10, the first structure 11 is displaced relatively tothe second structure 12, and a voltage corresponding to the appliedtorque is output from the first strain sensor 19. When the applicationof the torque to the torque sensor is removed, the first strain sensor19 is restored by elastic deformation.

On the other hand, as shown in FIG. 17B, when torque greater than therated torque is applied to the torque sensor 10, the side face of theconcave section 14 f of the first structure 11 is brought into contactwith the other end 16 b of the stopper 16, displacement of the firststructure 11 relative to the second structure 12 is limited.Accordingly, the first strain sensor 19 is protected within the range ofelastic deformation. When the application of the torque to the torquesensor 10 is removed, the first strain sensor 19 is restored by theelastic deformation. The second strain sensor 20 is also protected bythe same configuration.

FIG. 18 is a view for explaining a relationship between the torque to beapplied to the torque sensor 10 as the load and operation of the stopper16, and schematically shows a relationship between the torque to beapplied to the torque sensor 10 and detected strain (output voltage ofbridge circuit 50).

As shown in FIG. 18, when torque equal to or less than the rated torqueis applied to the torque sensor 10, regarding the strain body 41 of thefirst strain sensor 19 (second strain sensor 20), the first structure 11is displaced relatively to the second structure 12, and a voltagecorresponding to the applied torque is output from the first strainsensor 19 (second strain sensor 20).

On the other hand, when torque greater than the rated torque is appliedto the torque sensor 10, the side face of the concave section 14 f isbrought into contact with the stopper 16, deformation of the pluralityof third structures 13 is suppressed by the stiffness of the stopper 16(stopper 17) and, concomitantly with the suppression, deformation of thestrain body 41 is suppressed. That is, the operating point Op of thestopper 16 is set to a value equal to the rated torque, and stopper 16protects the strain body 41 against torque greater than the ratedtorque.

Advantage of Third Embodiment

According to the third embodiment described above, the stopper 16(stopper 17) serving as the cover is provided on each of the firststrain sensor 19 and second strain sensor 20, the one end 16 a of thestopper 16 is fixed in the part of the concave section 14 f on thesecond structure 12 side and, when torque greater than the rated torqueis applied to the torque sensor 10, the other end 16 b is brought intocontact with the side face of the concave section 14 f on the firststructure 11 side. Accordingly, it is possible to protect the firststrain sensor 19 and second strain sensor 20. Furthermore, thestructures other than the first strain sensor 19 and second strainsensor 20 are also protected from plastic deformation or the like as inthe case of the first strain sensor 19 or second strain sensor 20.

Moreover, it is possible to make the rated torque of the torque sensor10 closer to the 0.2% proof stress of the strain gage. Accordingly, itis possible to enhance the output voltage of the bridge circuit 50 atthe rated torque. Accordingly, it is possible to provide ahigh-resolution high-accuracy torque sensor.

FIG. 19 is a view showing a relationship between the strain and stressof a strain gage, and shows the rated torque of the torque sensoraccording to the third embodiment, and rated torque of a torque sensoras the comparative example having no stopper 16 and stopper 17.

In the case of a general torque sensor as the comparative example havingno stopper 16 and stopper 17, the strain gage is designed by setting thesafety factor for shock or fatigue to about 3 to 5. When the safetyfactor is made, for example, 3, the stress of the strain gage is set toone-third of the 0.2% proof stress. Accordingly, the rated torque isalso set to one-third of the breakdown torque.

Conversely, in the case of the third embodiment, the first strain sensor19 and second strain sensor 20 are respectively protected by the stopper16 and stopper 17, and hence there is no need to set the safety factorof the strain gage to 1 or more. Accordingly, it is possible to set therated torque of the strain gage to a value greater than the generaltorque sensor having no stopper 16 and stopper 17. Accordingly, it ispossible to provide a high-resolution high-accuracy torque sensor.

Furthermore, by enhancing the stiffness of the stopper 16, it ispossible to provide a high-allowable load (high-maximum torque) torquesensor.

Modification Example

FIG. 20 is a view showing a first modification example of the thirdembodiment. In the third embodiment, the stopper 16 protects the firststrain sensor 19 by being brought into contact with the side face of theconcave section 14 f on the first structure 11 side at the other end 16b thereof.

In the first modification example, the other end 16 b of the stopper 16includes an opening 16 b-1 and, on the first structure 11 side of thefourth structure 14, a projection 14 g inserted in the opening 16 b-1 isprovided. Between the opening 16 b-1 and projection 14 g, a gap GP1 isprovided. The dimension of the gap GP1 is less than, for example, thedimension of the gap GP. Accordingly, when torque greater than theallowable torque is applied to the torque sensor 10, the projection 14 gis brought into contact with the opening 16 b-1 of the stopper 16,whereby it is possible to protect the first strain sensor 19.

The stopper 17 of the second strain sensor 20 also comprises aconfiguration identical to the stopper 16.

By the first modification example described above too, it is possible toobtain an advantage identical to the third embodiment. Moreover,according to the first modification example, the projection 14 g isbrought into contact with the opening 16 b-1 of the stopper 16, wherebyit is possible to more securely protect the first strain sensor 19(second strain sensor 20).

FIG. 21 shows a second modification example of the third embodiment.

Whereas the third embodiment comprises the stopper 16 and stopper 17,the second modification example, furthermore, comprises four stoppers16-1, 16-2, 17-1, and 17-2. The structure of each of the stoppers 16-1,16-2, 17-1, and 17-2 is identical to the stoppers 16 and stopper 17.

By the second modification example too, it is possible to obtain anadvantage identical to the third embodiment. Moreover, according to thesecond modification example, the number of stoppers is greater than thethird embodiment, and hence it is possible to more securely protect thefirst strain sensor 19 and second strain sensor 20.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A torque sensor comprising: a first structure; asecond structure; a plurality of third structures connecting the firststructure and the second structure; and at least one strain sensorconnected between the first structure and the second structure; whereinthe at least one strain sensor comprises a rectangular strain bodyconnected between the first structure and the second structure andincluding a surface parallel to plane surfaces of the first structureand the second structure, and a plurality of sensor elements provided onthe surface of the rectangular strain body, the plurality of sensorelements are disposed in a region of one side of each of the firststructure and the second structure with respect to a longitudinalcentral portion of the rectangular strain body, and the region on theone side is a region where a strain of the rectangular strain bodyagainst a force in a torque direction is equal to a strain of therectangular strain body against a force in a torque-excepted direction.2. The torque sensor of claim 1, wherein the first structure is annular,and the second structure is annular, the second structure is arrangedconcentrically inside the first structure, and the plurality of sensorelement provided on the rectangular strain body of the strain sensor aredisposed in an area on a side of the second structure with respect tothe longitudinal central portion of the rectangular strain body.
 3. Thetorque sensor of claim 1, wherein the strain sensor comprises a bridgecircuit including the plurality of sensor elements.
 4. The torque sensorof claim 2, wherein the strain sensor comprises the bridge circuitincluding the plurality of sensor elements.
 5. The torque sensor ofclaim 2, wherein the at least one strain sensor is provided in asymmetric position with respect to a center of each of the firststructure and the second structure.